a-Si, CdTe and CIGS solar cells are being investigated worldwide as new generation thin-film solar cells to succeed c-Si solar cells. Among them, the CIGS solar cell has established itself as a leader of next generation solar cells because of its high optical absorption coefficient and cell efficiency. However, the supply of the rare earth metal indium for the CIGS production can be a problem in the near future. Consequently, lots of researches have recently been focused on the development of indium-free solar cells. CZTS quaternary semiconductor compound is an ideal candidate for use in thin-film solar cells as all the constituent elements are earth abundant and environmentally benign.
Various synthesis techniques have been developed for the preparation of the light-absorbing layer (J. Photoenergy, 2011, 2011, Article ID. 801292). Katagiri et al. reported a sulfur-based CZTS solar cell with conversion efficiency of 6.75% (0.15 cm2) started from sputtered precursor layers of Cu, ZnS, and SnS and followed by in-line sulfurization for 3 h at 580° C. (Thin Solid Films, 2009, 517, 2455). By thermal co-evaporation, Wang et al. and Shing et al. described a method to prepare sulfur-based CZTS solar cells with conversion efficiencies of 6.8% (0.45 cm2) and 8.4% respectively after anealing at around 500° C. (Appl. Phys. Lett., 2010, 97, 143508 and Prog. Photovolt: Res. Appl., 2011, DOI: 10.1002, 1174). For the Cu2ZnSn(S,Se)4, Agrawal et al. (Purdue University) reported the fabrication of a solar cell using Cu2ZnSn(S,Se)4 nanocrystals via a robust film coating method with 7.2% (0.47 cm2) of conversion efficiency (J. Am. Chem. Soc., 2011, 132, 17384). 10.1% conversion efficiency of a 0.44 cm2 Cu2ZnSn(S,Se)4 solar cell has been obtained from a hydrazine-based solvothermal method reported by Barkhouse et al. from IBM (Prog. Photovoltaics: Res. Appl. 2011, DOI: 10.1002/pip. 1160). They suggested that the addition of selenium to the absorber layer results in a lower bandgap, where ultimately help to get a high current density. For the methods described above, either expensive deposition techniques (e.g. evaporation and sputtering) or highly toxic and flammable chemicals (e.g. hydrazine) are used in the fabrication processes. In industrial scale production, the use of high-temperature and high-vacuum equipment will increase the cost as well as the energy consumption. Besides, the use of large amount of toxic chemicals is detrimental to the environment.
Electrochemical deposition is one of the low-cost and simple processes to deposit a CZTS layer on a substrate. This method does not use toxic chemical. The electrolytic bath can be reused for a long period of time. Electrochemical depositions of copper, zinc and tin are done at a relatively low temperature, which does not require large amount of energy, as opposed to high temperature and high vacuum evaporation and sputtering techniques. There are few research groups working on electro-synthesis of CZTS thin films for solar cell application and none of them uses the current invention to prepare electroplated CZTS light-absorbing layer. Ennaoui et al. (Helmholtz-Zentrum Berlin für Materialien und Energie GmbH) reported the preparation of a Cu2ZnSnSe4 layer using H2Se as the Se sources in the annealing process (Thin Solid Films, 2009, 517, 2511). However, the use of highly toxic H2Se gas in CZTS manufacturing process is generally not recommended in large scale production purpose. Scragg et al. (University of Bath) reported the preparation of a Cu2ZnSnS4 layer using sulfur vapor as the sulfur sources under vacuum conditions in the annealing process (J. Electroanal. Chem., 2010, 646, 52). However, the method is designed only for a small substrate, and cannot be directly scaled up as a complicated sealing and annealing process under controlled-vacuum conditions is needed. In the above reports, they all carried out their studies on a small substrate, and they did not deposit Se and/or S layer using electroplating method. Therefore, a simple, low-cost, less toxic, and safer process for manufacturing large-area CZTS solar cell is therefore needed.
In US patent application publication number US20120061790A1, a p-type CZTS absorber was formed by using electrodeposition. One of the embodiments in '790 disclosed a copper zinc selenium (Cu—Zn—Se) alloy stack which was electrodeposited on a substrate in a plating bath, followed by electroplating a Sn layer. The Se annealing step was conducted at a temperature of from about 80° C. to about 100° C. for duration of about 30 minutes to about 60 minutes. However, this method involved an additional annealing step, i.e., in addition to the soft anneal and the final anneal, after the Se layer was plated onto the stack. The as-plated Se layer was amorphous and resistive and thus required an additional Se annealing step to make the layer crystalline and conductive. After that, the metal-semiconductor alloy stack was subjected to an intermediate/soft anneal which helped form the copper zinc selenium tin (Cu—Zn—Se—Sn) alloy followed by a final anneal in a S environment (to produce CZTS in the form of Cu2ZnSn(S/Se)4). A soft anneal was required to avoid formation of unwanted secondary phase, for example Cu2(S/Se)Cu2-X(S/Se), Sn(S/Se)2, Sn2(S/Se)3, Cu2Sn(S/Se)3, Zn(S/Se), etc. with single phase polycrystalline CZTS which eventually cause adverse effects on the efficiency of the cell. The problems of the method disclosed in '790 include: selenization requires long time during the disclosed Se annealing step; selenium attacks the substrate and causes an increase in surface roughness during long-time selenization. Also, an additional soft annealing step is required in '790. To solve these problems, the present invention provides a method which does not require long time in selenization and can avoid selenium attack to the substrate.